The little white boxes with horizontal slats are a common sight at weather stations in the UK and around the world.
The louvered housing for a thermometer to measure air temperature was invented in the 1860s by Thomas Stevenson, a lighthouse engineer and father of the author Robert Louis Stevenson. What is now widely known as the Stevenson Screen was originally described by its developer simply as a “box to put a thermometer in.”
Indeed, the Stevenson Screen was a brilliant innovation. Its simple and easily replicable design allowed outside air to flow around the thermometer, protecting it from direct sunlight and rain that could affect the reading. The Stevenson Screen also made the job of meteorologists easier, since they no longer had to periodically remove the shade or perform any other tasks beyond reading the thermometer itself. The Stevenson Screen was widely adopted and, in its various modern forms, is perhaps the best-known device for monitoring the weather.
The thermometers in these boxes must have dutifully recorded very high temperatures during the UK's recent heatwave, but after 160 years of use, is it time for the Stevenson Screen to be retired?
In the new study, my meteorologist colleague Steven Burt and I compared temperatures measured according to traditional methods with those obtained using the latest system.
Some small but obvious inconsistencies did appear, especially when winds were calm overnight or on clear winter days. Nevertheless, the Stephenson Screen continued to be remarkably effective at providing accurate and continuous measurements for over 150 years.
Aitken effect
One of the limitations of the Stevenson Screen was already pointed out by Scottish meteorologist John Aitken in 1884. Aitken argued that the screen did not work well on calm, sunny days because it had no natural ventilation.
Although no solution to this problem has been widely implemented, the latest generation of thermometer screens incorporate electric fans to force air through them. These are known as suction thermometers. In the United States and Canada, a network of suction thermometers has appeared, which are quite different from Stevenson screens and may be cylindrical or pole-mounted.
Aspirator thermometers are rare in the UK but we are likely to see more of them in the future. But before they switch, meteorologists need to be sure they perform roughly the same role. Making any big change to how we measure temperature needs to be done carefully and slowly to ensure we keep a consistent record of the climate.
Running a modern aspirator thermometer alongside a Stevenson screen is one way of identifying differences in measurements. We did this over a three-year period at the University of Reading's Atmospheric Observatory using an automated system that generated over 1.5 million temperature records.
We were particularly interested in comparing how the two methods recorded daily maximum and minimum temperatures. Observing these extreme temperatures in one location can help identify local climate changes.
Overall, the differences were surprisingly few and small: in only 1% of cases were there differences of more than 1°C. Across Europe, it is estimated that around 4% of maximum temperatures and 12% of minimum temperatures are affected in this way. This is relatively small and does not affect the important conclusion that temperatures are rising globally.
However, we found that these differences do not only occur on sunny days, but are also evident at night. When the night-time wind is calm or weak, the Stevenson screen cools down and the minimum temperature recorded is lower than the temperature on the aspirator thermometer. Conversely, daily maximum temperatures are recorded mainly when the wind is strong, but the Stevenson screen maximum temperature can still exceed the aspirator thermometer temperature. This mainly happens in winter, when the sun is low on the horizon, and occasionally daily maximum temperatures can become abnormally high.
Aitken's insight has been largely ignored until now, but he was ahead of his time: the suction thermometer, now recommended by the World Meteorological Organization, overcomes the limitations of the Stevenson screen in light or no wind conditions.
However, despite being a device that is over 150 years old, the Stevenson screen is still remarkably effective.
This article was originally published on The Conversation by Giles Harrison from the University of Reading. Read the original article here.
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